A transparent conductive film, because of having high conductivity and high transmittance in a visible light region, has been utilized in an electrode or the like, for a solar cell or a liquid crystal display element, and other various light receiving elements, as well as a heat ray reflection film for an automotive window or construction use, an antistatic film, and a transparent heat generator for various anti-fogging for a refrigerator showcase and the like.
As a well known practical transparent conductive film, there has been included a thin film of tin oxide (SnO2)-type, zinc oxide (ZnO)-type, indium oxide (In2O3)-type. As the tin oxide-type, the one containing antimony as a dopant (ATO), or the one containing fluorine as a dopant (FTO) has been utilized, and as the zinc oxide-type, the one containing aluminum as a dopant (AZO), or the one containing gallium as a dopant (GZO) has been utilized. However, the transparent conductive film most widely used industrially is the indium oxide-type. Among them, indium oxide containing tin as a dopant is called an ITO (Indium-Tin-Oxide) film, and has been utilized widely, because, in particular, a film with low resistance can be obtained easily.
The transparent conductive film with low resistance is suitably used widely in a surface element or a touch panel or the like, of such as for a solar cell, a liquid crystal, an organic electroluminescence and an inorganic electroluminescence. As a production method for the above various transparent conductive films, a sputtering method, a vacuum deposition method, or an ion plating method has been known.
A sputtering method among them is an effective method in film formation of a material with low vapor pressure, or in requiring precise film thickness control, and because of very simple and easy operation thereof, it is widely utilized industrially. In a sputtering method, a target for sputtering is used as a raw material of a thin film. The target is a solid material containing a constituent element of the thin film to be formed, and a sintered body such as a metal, a metal oxide, a metal nitride, a metal carbide, or in certain cases, a single crystal is used. In this method, in general, after making high vacuum once with a vacuuming apparatus, rare gas (argon or the like) is introduced, and under a gas pressure of about 10 Pa or lower, a substrate is set as an anode and a target is set as a cathode to generate glow discharge between them and generate argon plasma, and argon cations in the plasma are collided with the target of the cathode, and particles of the target component flicked thereby are deposited on the substrate to form a film.
The sputtering method is classified, based on a generation method for argon plasma, and the one using high frequency plasma is called a high frequency sputtering method, while the one using direct-current plasma is called a direct-current sputtering method. In general, the direct-current sputtering method provides faster film formation speed, lower cost of power source facility, and simpler film formation operation, as compared with the high frequency sputtering method, and by these reasons, it has been used widely industrially.
However, in recent years, an ion plating method has been noticed as a method which is capable of forming a transparent conductive film having equivalent or better quality as compared with a direct current sputtering method. The ion plating method is a method for evaporating a raw material called a tablet (or a pellet) composed of a metal or a metal oxide, by resistance heating or electron-beam heating, under a pressure of about 10−3 to 10−2 Pa, and further activating the evaporated substance using plasma along with reaction gas (oxygen) to deposit it on a substrate. In particular, an ion plating method using a pressure-gradient-type plasma gun utilizes direct-current arc discharge of a large current, therefore it is capable of generating high density plasma having characteristics that evaporation speed of a sample is higher as compared with a conventional direct-current sputtering method. Conventionally, there has been a defect that uniform film formation onto a large area substrate is difficult, caused by irregular distribution of film quality or film thickness, however, it has been overcome by a technology adjusting a magnetic field at the vicinity of a Haas where plasma beams inject, of, for example, PATENT LITERATURE 1, by which uniform film formation onto a large area substrate has become possible.
It is preferable to use the oxide sintered body, as a tablet for ion plating to be used in formation of the transparent conductive film, similarly as in the target for sputtering, and using this, the transparent conductive film having a constant film thickness and constant characteristics can be produced stably. It should be noted that the tablet for ion plating uses the one having a low sintering density of about 70%, for example, as described in the NON PATENT LITERATURE 1, to avoid fracture caused by electron beam heating, different from the tablet for sputtering. The case of too high or too low density tends to generate crack or fracture onto the oxide sintered body, leading to damage. In addition, the oxide sintered body tablet is required to uniformly evaporate, and thus it is preferable that a substance having stable chemical bond and being difficult to evaporate does not exist together with a substance which is easy to evaporate and present as a main phase.
In addition, a method for forming a thin film by vaporization and ionization of the oxide sintered body, which is an evaporation material (tablet), by an ion plating method, has a problem of generating splash of the evaporation material in heating, leading to a pin-hole defect in a deposited film, caused by scattering particles. “Splash” means the following phenomenon. That is, when the evaporation material is heated by irradiation of plasma beams or electron beams in vacuum, it is vaporized at a time when certain temperature is reached, and it initiates uniform evaporation in an atomic state. “Splash” means a phenomenon where visible sized splash having a size of about several μm to 1000 μm, jumps out of the evaporation material, by being mixed in uniform evaporation gas, and collides onto the deposited film, in this occasion. Generation of this phenomenon causes to incur the pinhole defect in the deposited film by collision of the splash, which not only impairs uniformity of the deposited film but also significantly deteriorates performance as a conductive film.
As described above, use of an oxide target, which is difficult to generate splash of the evaporation material in heating, and does not generate the pinhole defect in the deposited film by the scattering particles, can be said important, to form the transparent conductive film of an oxide such as ITO, by an ion plating method.
Now, in recent years, a solar cell module using the transparent conductive film different from the ITO has been proposed. In PATENT LITERATURE 2, there has been proposed a solar cell module superior in weatherability, and described that by using indium oxide added with tungsten as the transparent conductive film, arithmetic average roughness (Ra) can be deceased, and electric conductivity and light transmission property can be enhanced in addition to weatherability, and described that, for example, by using a sintered body of In2O3 powder mixed with 3% by weight of WO3 powder, as a target (a tablet or a pellet), the transparent conductive film was formed by an ion plating method. In PATENT LITERATURE 2, although it is said that the transparent conductive film composed of indium oxide added with tungsten formed by an ion plating method, is useful as the transparent conductive film for a solar cell module, there is no detailed description on a tablet composed of the evaporation material, which is the raw material thereof, that is, the oxide sintered body.
In PATENT LITERATURE 3, there has been proposed a sintered body target obtained by sintering mixed powder having an indium oxide as a main component, and containing tungsten within a range of 0.003 to 0.15, as tungsten/indium atomic ratio. However, PATENT LITERATURE 3 has only disclosed a high density sputtering target, having a uniform texture composed of the indium oxide phase with a bixbyite type structure, and a production method thereof, thinking that it is preferable that tungsten makes a solid solution by substitution at an indium site of indium oxide, and it is preferable that relative density is 90% or higher. That is, it is a high density sputtering target, therefore when it is used as a tablet for ion plating, it generates crack, fracture or splash, and thus desired film formation cannot be continued.
In PATENT LITERATURE 4, there has been proposed the oxide sintered body mainly composed of indium, containing tungsten and having a specific resistance of 1 kΩcm or smaller, or the oxide sintered body mainly composed of indium, containing tungsten and tin, having a specific resistance of 1 kΩcm or smaller, and it has been described desirable that content of tungsten is desirably 0.001 to 0.17 as W/In atomic ratio, and further it has been described desirable that it is mainly composed of the indium oxide crystal phase with a bixbyite type structure, where tungsten is present as a solid solution and/or an indium tungstate compound phase, and the crystal phase of tungsten oxide does not exist. In addition, also PATENT LITERATURE 4, similarly as PATENT LITERATURE 3, has described mainly a sputtering target, however, crack, fracture or splash cannot be suppressed, even if a high density sputtering target is used as the tablet for ion plating.
As described above, in conventional technology relating to the oxide sintered body containing indium and tungsten, there has not been studied sufficiently on prevention of crack, fracture or splash, in film formation by ion plating, which becomes important in mass production of a crystalline transparent conductive film, and thus there has been desired the advent of the oxide sintered body containing indium and tungsten, which has solved these problems.